There are many applications for reference voltage generator circuits that are stable regardless of changes in ambient temperature and supply voltage. Such devices have application in oscillator, timer, and voltage regulation circuitry.
FIG. 1 is a circuit diagram of a conventional reference voltage generator circuit. As shown in the drawing, the conventional reference voltage generator circuit has a plurality of resistors R1.about.R4 serially coupled to a power source voltage Vcc. A plurality of NMOS transistors MN1.about.MN3 is coupled between the plurality of resistors R1.about.R4 and a ground voltage Vss. The plurality of NMOS transistors MN1.about.MN3 function as resistors.
A PMOS transistor MP1 compensates for the threshold voltage of the NMOS transistors MN1.about.MN3 according to temperature variations. The PMOS transistor MP1 controls the reference voltage Vref according to the voltage across the resistors R3 and R4 and the NMOS transistors MN2 and MN3. That is, when fuses f1.about.f4 are cut by a laser beam, for example, the power supply voltage Vcc is applied to the resistors R3 and R4 or the NMOS transistor MN2 and MN3 such that the voltage at node N1 is lowered. If the voltage at node N1 is lower, the gate voltage of the PMOS transistor MP1 is lowered and the PMOS transistor MP1 operates weakly. Thus, the reference voltage Vref is controlled. The plurality of fuses f1.about.f4 is parallely coupled to each resistor R3 and R4 and each NMOS transistor MN2 and MN3. The fuses f1.about.f4 are selectively cut by the laser beam in order to obtain the desired reference voltage Vref.
The reference voltage Vref is changed by various factors present during the manufacturing process like temperature variation that make it difficult to accurately cut the fuses.
When a wafer goes through electric die sorting (EDS), the reference voltage Vref is typically compared with a target voltage. The target voltage is the desired design voltage. After comparing the two voltages, the fuses are cut by the laser beam when the two voltages are the same.
Thus, the EDS operation is performed in two steps: a measuring step wherein various parameters such as the reference voltage is measured and a determining step wherein the device is passed or failed after the fuse is shut off based on the various parameters.
After performing the above described two steps, the device is repaired. As a result, the EDS process increases the total sorting time because the device is tested several times with the incorrect shut-off of the fuse thereby generating a low device yield and a consequent high cost.